Read examples of ballads. Besides being fun, they'll give you a feel for the rhyme and rhythm that's a big part of what makes the poem a ballad. My Song Book (see link in Resources) is a German website, but has an extensive alphabetical list of ballads in English.
Choose a story to tell. It should have lots of action and a straightforward structure with clear conflict, tension buildup and a satisfying resolution.
Decide on a rhyming scheme for your ballad. The second and fourth lines of each stanza have to rhyme, but it's you're choice if you want an "abab" or an "abcb" structure. Use a rhyming dictionary or Rhymezone (see link in Resources) to help you.
Use 6 or 8 syllables for your rhythm scheme. Iambic tetrameter is easier with eight syllables in a "da DUM" rhythm as in "Come LIVE with ME and BE my LOVE" (Marlowe). For a greater challenge, use a six-syllable line like Roethke: "Slid FROM the KITchen SHELF."
Make your ballad a funny story. These poetic forms have a long tradition of bawdy humor from when they were sold in the streets of London. They told of the mischief of Robin Hood and Puck.
Tap into folklore for inspiration. Many ballads from different countries have retold folktales to the delight of both children and adults. Local and ethnic legends and tales make great material and educate people about cultures they're not familiar with.
Write a love ballad and set it to music. Listen for ballads in present day songs from rock, jazz, country, new age and musicals.